Field of the Invention
The invention relates to the field of Z-pinch apparatus and methods for nuclear fusion for neutron.
Description of the Prior Art
Thermonuclear fusion energy production has yet to be demonstrated in the laboratory where the energy produced is greater than the energy injected to create the reaction. Several approaches are being pursued around the world.
The simple Z-pinch is a cylindrical plasma column that implodes to the axis of symmetry when subjected to a large, sustained-current pulse. A typical or conventional Z pinch load is constructed from a wire-array, foil, plasma jet, or gas-puff, or combinations thereof. When driven by a modern, low-inductance, high-voltage, pulse-power circuit, the pinch-current pulse can reach many megaamperes and the delivered power 100's of terawatts. Z-pinch plasmas with keV temperatures and near-solid densities are produced routinely. The Z Facility at the Sandia National Laboratory is perhaps the best example: it produces a 20 MA, 100 ns rise time, 100 TW current pulse and a conventional Z pinch that can radiate mega-joules of X-ray energy in a few nanosecond pulse. Such plasmas are of great scientific and technical interest, for example in studies related to fusion, atomic physics, laboratory astrophysics, etc.
The Z-pinch dynamics is comprised of three phases: implosion, stagnation, and disassembly. The implosion phase is when the discharge-current builds and the pinch is driven radially inward by the Jz×Bθ force, where Jz is the vector axial, plasma-current density and Bθ is the vector azimuthal, self-magnetic field. At the stagnation phase the pinch is confined briefly, typically for a few nanoseconds when the radial motion of the pinch has ceased, or nearly so, and the plasma is compressed to a high-energy density. Generally, the imploded mass and the initial-pinch radius are adjusted so that stagnation occurs after the current maximum.
At stagnation the implosion-kinetic energy and the inductive energy, stored local to the pinch, is rapidly converted into plasma-internal energy. Shock heating is important. The small radius of the pinch plasma at stagnation also increases its electrical resistance, enhancing the energy deposited by Ohmic heating. MHD instabilities occur in this phase: typically, for example the (m=0) sausage instability or the (m=1) kink instability.
A fusion burn will result if the Z-pinch remains stable for a sufficiently long time, while the required high temperature and density are sustained. Ignition is possible, if the fusion products are sufficiently well-confined. This will occur if the azimuthal-magnetic field is sufficiently intense that fusion α particles are confined, that is, ρα<<Rpinch, where ρα is the a particle gyro-radius and Rpinch is i the compressed pinch radius.
Following stagnation the pinch disintegrates rapidly, due to the rebound in plasma pressure and the accumulated effect of instabilities. Z pinches are susceptible to the Rayleigh-Taylor (RT) instability during implosion. Many techniques have been developed to control the effect of the RT instability, all directed toward maximizing the accumulated-pinch energy. The most common techniques consist of altering the load configuration to provide a more uniform, initial-mass distribution, or reducing the time needed to obtain a uniform, highly-conducting plasma at current initiation. Other approaches involve decreasing the rise time of the current pulse and using concentric, multi-layer mass distributions.
The gas-puff Z-pinch was developed in the 1970's as a stable alternative to the more widely used wire array Z-pinches and has demonstrated a surprisingly large range of scalability; having been implemented on short- and long-implosion time generators, with rise times, τ1/4≈0.1−1 μs and load currents, Iload≈0.1−10's of MA. Gas-puff pinches have also been configured to study gas mixtures. Gas-mixture Z-pinches have demonstrated a unique ability to produce a higher energy radiated spectrum and higher X-ray yield, than a Z-pinch of either gas imploded separately. Multi-layer gas-puff implosions have also produced better results than single layer, or uniform-fill Z-pinch.
The improvements observed for gas mixtures and multi-shell implosions suggests that there is a complex interplay of shock-driven compression heating, current-diffusion, flux-compression, and radiation-transport at work, for which further analysis will provide deeper insights. Staging the implosion, to optimize these dynamical processes, is expected to have specific benefits for fusion. A gas-puff mixture of deuterium and argon was tested recently, with a reported neutron yield of, Y≈3.7-3.9×1013; modeling suggests that the neutrons are thermonuclear.